An international collaborative study was carried out to determine the suitability of the current WHO II - IX - X concentrate standard, 84/681, for assigning potency to the more highly purified Factor IX concentrates. Three Coagulation Factor IX (Human) preparations and one Factor IX Complex preparation were assayed by the one stage method against WHO 84/681 following predilution to 1.0 u/ml in buffer, 1% albumin, or factor IX deficient plasma. There were no cases of non-parallelism between any of the preparations and the current WHO standard. Predilution of the Coagulation Factor IX (Human) preparations in 1% albumin or factor IX deficient plasma gave similar potency values. Predilution in buffer gave significantly lower (P<0.01) potency values. For the Factor IX Complex preparation, potency estimates were significantly different (p<0.01) with each prediluent. The overall precision was similar within each predilution for all preparations with predilution in buffer being less precise than predilution in albumin or in deficient plasma. WHO standard 84/681 appears to be a suitable standard for the potency determination of the more highly purified factor IX preparations. Predilution in 1% albumin or factor IX deficient plasma is recommended as they give equivalent results with the least variability.